Is the prion structure solved?
We report here on the current knowledge on the nature of the scrapie agent or prion. Several lines of evidence suggest that the abnormal isoform of prion protein (PrP) is crucial for scrapie infectivity while evidence that PrP is also a part of the entire particle of the scrapie agent (prion) is much weaker. There is no doubt, however, that conformational changes (transitions from alpha-helical into beta-pleated structures) of PrP underlay scrapie pathogenesis. In view of the notorious puzzling nature of the scrapie agent, the electron microscopic search for the ultrastructural correlate of it is still warranted. Thus, we discuss the nature of tubulovesicular structures (TVS), the only diseases-specific particles known so far and the association between TVS and PrP fibrils which was recently discovered.